(a) Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more particularly, to a method for manufacturing a semiconductor device in which tantalum oxide film is used for an insulating film in a capacitor element.
(b) Description of the Related Art
An attempt has been made to employ an insulating film having a higher dielectric constant in capacitor elements of memory cells in a VLSI, to be developed as a dynamic random access memory (DRAM) device of a 256 Mbit storage capacity or more, since such an insulating film is capable of effectively increasing the capacitance per unit area in the VLSI.
Of the insulating films having a higher dielectric constant, a tantalum oxide film formed by chemical vapor deposition (CVD) has been intensely studied. This is because the tantalum oxide film has a large dielectric constant ranging from 25 to 30 and has an excellent step-coverage characteristic among other materials having higher dielectric constants, and furthermore, it is more easily deposited than the other materials.
FIGS. 1A and 1B are partial sectional views of a semiconductor device showing successive steps in a conventional method for manufacturing a stacked capacitor element in a DRAM cell. In FIG. 1A, the lower electrode 2 of a stacked capacitor element made of polysilicon is connected to an active region 51A of a transistor element by way of a through-hole 57 formed in a first interlayer insulating film 47 and a second interlayer insulating film 48 overlying the transistor element including a bit line 56.
A tantalum oxide film 11 is formed on a lower capacitor electrode 2 by a low pressure chemical vapor deposition (LPCVD) technique using pentaethoxytantalum (Ta(OC.sub.2 H.sub.5).sub.5) as a source gas. The tantalum oxide film 11 is then thermally treated at a high temperature in an oxygen atmosphere for reducing the leakage current through the tantalum oxide film 11. The thermal treatment is conducted at a temperature ranging from 700.degree. to 900.degree. C. Subsequently, an upper capacitor electrode 3 made of tungsten (W) is formed on the tantalum oxide film 11 (see FIG. 1B). A capacitor element having the lower electrode 2, insulating film 11 and upper electrode 3 is thus formed in the DRAM device.
The capacitor element formed by the conventional method as described above has a drawback in which the capacitor element thus obtained does not have enough capacitance per unit area Cs (Cs=11.5 fF/.mu.m.sup.2), which is only equivalent to that of about 3 nm (nanometer) in terms of the thickness of the SiO.sub.2 film. The capacitance per unit area Cs of the tantalum oxide film is not enough for a capacitor element of a 256 Mbit DRAM device. Moreover, the capacitor element formed by the conventional method has a drawback in which a leakage current having a current density about 10.sup.-8 A/cm.sup.2 flows across the tantalum oxide film when only a low voltage of about 0.7 V is applied between the upper capacitor electrode 3 and the lower capacitor electrode 2, the amount of the leakage current being not acceptable in a DRAM device. Additionally, the leakage current tends to increase through subsequent thermal treatments at high temperatures, such as activation of implanted ions and fellow of insulating films.